Aqueous emulsions containing copolymers of vinyl esters and ethylene, and especially vinyl acetate and ethylene (VAE copolymers) are well known as binders for paints and for adhesives for use in the wood, paper and packaging industries. It is also known to use vinyl ester/ethylene emulsion copolymers in powder form in self-leveling flooring screeds and ceramic tile adhesives. See, for example, U.S. Pat. No. 6,716,941 discussed in more detail below.
For an increasing number of applications, industry standards and/or government regulations require that emulsion copolymers be very low in residual monomer content and content of volatile organic compounds (VOCs). One VOC component which is frequently singled out for specific control is formaldehyde, since formaldehyde is suspected to be a human carcinogen and can be produced by decomposition of some components which are conventionally used in emulsion polymerization processes and products.
One potential source of formaldehyde in emulsion polymerization is the redox initiator system, since a common reducing agent used in such systems is sodium formaldehyde sulfoxylate, which can dissociate to form free formaldehyde. There is therefore significant interest in developing new redox initiator systems for emulsion polymerization which minimize or avoid the generation of formaldehyde, without impairing the properties of the resultant polymer. For example, in the case of use in self-leveling flooring screeds, it is important that the vinyl ester/ethylene copolymer exhibits good adhesive and flow properties and shows early strength on curing.
For example, U.S. Pat. No. 6,635,725 discloses a method for producing an ethylene-vinyl acetate copolymer aqueous emulsion comprising co-polymerizing monomers comprising ethylene and vinyl acetate by using a formaldehyde-free redox catalyst comprising a transition metal salt and an erythorbic acid, as the reducing agent thereof, and 1.8 to 10 mol of hydrogen peroxide, as an oxidizing agent thereof, per one mol of the erythorbic acid. The use of such a controlled amount of hydrogen peroxide is said to be necessary to avoid discoloration of the ccopolymer which is used as a binder for a paper coating composition.
According to the present invention it has now been found that a redox-system comprising a reducing agent based on erythorbic acid and/or sulfinic acid in combination with t-butyl hydroperoxide, as an oxidizing agent, is an effective formaldehyde-free initiator for the emulsion polymerization of a vinyl ester/ethylene monomer mixture. Moreover, when dehydrated into powder form, the resultant copolymer has excellent properties for use in self-leveling flooring screeds and ceramic tile adhesives.
U.S. Pat. No. 6,716,941 discloses a process for forming an ethylene-vinyl acetate copolymer dispersion in the presence of a surfactant-free stabilizing system consisting of polyvinyl alcohol, and in the presence of from 0 to 10 percent by weight of a seed polymer based on the weight of the dispersion, wherein said copolymer dispersion has a solids level of greater than 65 percent by weight, and a viscosity of less than 5000 mPa·s when measured at 55 percent solids at 25° C. Suitable polymerization initiators are said to include hydrogen peroxide, sodium persulfate, potassium persulfate, ammonium persulfate, and t-butyl hydroperoxide, which can be used alone or together with reducing agents such as sodium formaldehyde-sulfoxylate, iron-II-salts, sodium dithionite, sodium hydrogen sulfite, sodium sulfite, sodium thiosulfate, ascorbic acid, and erythorbic acid. In the Examples a redox couple comprising sodium formaldehyde sulfoxylate and t-butyl hydroperoxide is used. Spray drying of the resultant copolymer dispersion is said to produce a free flowing redispersible powder useful in floor screeds and ceramic tile adhesives.